1. Field of the Invention
The present invention relates to a RF antenna and a microwave antenna, and more particularly to an electrically small planar antenna matched with an electronic chip of RFID (Radio Frequency Identification) and /or a wireless sensor transponder.
2. Description of the Related Art
At UHF-frequencies and in the L-band the size of even a single-half-wave dipole antenna is precluded in many mobile and Radio Frequency-Identification (RFID) applications. So small (relative to wavelength) antennas are in very high demand. However, the size of the antenna for a given application is not related mainly to the technology used, but is determined by well-known laws of physics. Namely, the antenna size with respect to the wavelength is the parameter that has the prevalent influence on the radiation characteristics.
All antennas are used to transform a guided wave into a radiated one, and vice-versa. Basically, to perform this transformation efficiently, the antenna size should be of the order of a half wavelength or larger. Of course, antenna can be smaller, but at expense of bandwidth, gain, and efficiency. So the art of antenna miniaturization is always an art of compromise among size, bandwidth, and efficiency.
As regards theoretical studies of antenna miniaturization, please refer to the following literature cited. [H. A. Wheeler, “Fundamental Limitations of Small Antennas,” Proceedings of the IRE, vol. 35, pp. 1479-1484, December 1947; L. J. Chu, “Physical Limitation on Omni-Directional Antennas,” Journal of Applied Physics, vol. 19, pp. 1163-1175, December 1948; and R. F. Harrington, “Effect of Antenna Size on Gain, Bandwidth and Efficiency,” Journal of Research of the National Bureau of Standards—D. Radio Propagation, vol. 64D, pp. 1-12, January-February 1960].
According to these initial studies, the small antennas are constrained in their behavior by a fundamental limit: the smaller the maximum dimension of the antenna, the higher its Quality Factor (Q), or equivalently, the narrower its bandwidth. The computation of the smallest possible Q for a linearly polarized antenna was refined by McLean [J. S. McLean, “A Re-examination of the Fundamental Antenna Limits on the Radiation Q of Electrically Small Antennas,” IEEE Transactions on Antennas and Propagation, vol. 44, pp. 672-676, May 1996].
Accordingly, the art of antenna miniaturization always requires a compromise among the size, bandwidth and efficiency (i.e., gain) of the antenna. In the case of a planar antenna, if most of the antenna region takes part in radiation, the most superior compromising point can be found. That is, the antenna miniaturization technology requires the compromise among the size, bandwidth and efficiency of the antenna.
An original way to make an antenna smaller than resonant size and yet keeping resonant features such as relatively high gain and efficiency is disclosed in WIPO Publication WO 03/094293. FIG. 1 illustrates the antenna disclosed in WO 03/094293.
Referring to FIG. 1, the antenna 1 includes a dielectric substrate 2, a feeder 5, a metal layer 3, a main slot 4 and a plurality of sub-slots 6a to 6d formed in pattern on the metal layer 3. The metal layer 3 that includes the main slot 4 and the sub-slots 6a to 6d forms a radiation part of the antenna 1.
Additionally, FIG. 2A is a view illustrating a radiation part of a conventional antenna having straight-line terminating slots, FIG. 2B is a view illustrating a radiation part of a conventional antenna having turn terminating slots, and FIG. 2C is a view illustrating a radiation part of a conventional antenna having a spiral terminating slots.
In FIGS. 2A to 2C, the same drawing reference numerals are used for a main slot and a metal layer that are the common constituent elements. A plurality of sub-slots 8a to 8d, 9a to 9d, and 10a to 10d having diverse shapes may be formed on each end part of the main slot 4.
The conventional antennas as described above, however, have the drawback in that their bandwidths are generally narrow. In diverse application fields, the small operating frequency bandwidth of a small antenna causes serious problems. Accordingly, it is preferable to provide a small antenna that can operate over an enhanced bandwidth without affecting the radiation pattern, gain and polarization purity of the antenna.
Meanwhile, an RFID (Radio Frequency Identification) transponder is a responsive tag appliance that transmits the contents of a built-in memory through a backscatter communication with an interrogator or a reader. A passive RFID transponder is not provided with a battery, but obtains all necessary energy from a carrier signal of a reader instead. A passive wireless sensor appliance includes a semiconductor chip (for example, ASIC (Application Specific Integrated Circuit)) connected to an antenna. Practically, a low-priced planar antenna and/or wireless sensor transponder for the RFID having a small electrical size has become a matter of great concern. Recently, even an antenna having a size of ¼ of a wavelength is excluded from many application fields.
However, the implementation of the small antenna in the RFID and/or wireless sensor transponder design causes another problem in that the semiconductor chip of the transponder essentially has a complex input impedance having a capacitive reactance. Accordingly, in order to operate the antenna in the bandwidth of an RFID system, the problem of the complex conjugate matching between the transponder antenna and the semiconductor chip should be solved.
The impedance matching between the semiconductor chip of the transponder and the antenna is important to the whole performance of the RFID system. That is, the mismatching exerts an important effect upon the maximum operation distance between the interrogator and the transponder. Due to specified safety regulations and other legislations, the power radiated from the interrogator is somewhat limited. But, a passive RFID transponder obtains the driving power by rectifying an interrogation signal delivered to the chip by the antenna.
A rectifying circuit is a part of the semiconductor chip such as ASIC, is provided with a number of diodes (for example, Schottky diodes) and capacitors, and substantially give rise to a complex input impedance having a capacitive reactance. Typically, the impedance of the semiconductor chip has several to several tens of active ohms and several hundreds of reactive ohms. Accordingly, the ratio of the resistance to the reactance is very high.
In the above-described situations, the conventional matching technology is implemented by an additional external matching circuit based on an inductor. However, this conventional method has a new problem in that its manufacturing cost is ridiculously increased. Additionally, this separation type matching circuit greatly reduces the performance of the system. Accordingly, the impedance of the antenna should directly match the semiconductor chip of the transponder.
Generally, a circuit that includes an antenna and a rectifying circuit is called a rectenna.
FIGS. 3A to 3F are views illustrating the conventional transponder antennas. The typical transponder antennas have a planar structure formed with metal strip patterns.
FIG. 3A shows a conventional half-wavelength dipole antenna. The impedance of the half-wavelength dipole antenna is matched to the impedance of the rectifier by lowering the radiation resistance of the antenna by parallel metal strips and increasing the reactance by a small loop. As described above, the half-wavelength antenna is excluded from many application fields. Another example of a half-wavelength antenna is illustrated in FIG. 3B. The impedance of the antenna illustrated in FIG. 3B is matched by two separated coils.
FIG. 3C shows a folded half-wavelength dipole antenna having separated coils. The separated coils may be replaced by planar narrow meander strip patterns having an inductive property. The antennas illustrated in FIGS. 3B, 3C and 3D may suffer an additional loss caused by the separated coils or the narrow strip meanders.
FIGS. 3E and 3F illustrate small antennas in which a loop and a dipole structure are combined. [World Intellectual Property Organization Publication WO 03/044892 A1 (2003 May 30 Bulletin 2003/43) entitled “Modified Loop Antenna with Omnidirectional Radiation Pattern and Optimized Properties for Use in an RFID Device” by Varpula et al].
The important defect of the antennas illustrated in FIGS. 3E and 3F is a relatively small antenna RCS (Radar Cross Section). The RCS indicates the property about how much the antenna scatters the electromagnetic energy of an incident wave field. Since the modulated RCS is essentially used for the data transmission from the transponder to the reader, the RCS of the rectenna is very important to the backscatter communication.
Accordingly, it is preferable to provide a rectenna provided with an electrically small conjugate matched antenna that can operate with an enhanced RCS all over increased bandwidth without affecting the radiation pattern, efficiency, polarization purity, etc.